Origin of magnetite and apatite ores in the Esfordi magnetite-apatite ore deposit NE of Bafq, south Yazd: insights from mineralogy, geochemistry, microthermometry, O-H stable and U-Pb and Nd-Sm non-stable isotopes

Petrographic and mineralogical data indicate the widespread presence of five generations of apatite, two generations of monazite with minor xenotime in the Esfordi deposit. The O-H isotopic studies on the 1st- and 2nd-generations of apatites and massive fine-grained and vein-type apatites as well as their Sr and Mn contents, showed that the source of phosphorous was the sedimentary phosphorites. The ratio of 143Nd/144Nd vs 147Sm/144Nd and εNd vs P2O, and the difference of Nd isotopic ratios in the massive fine-grained and vein-type apatites indicate that they are not reproductively related to the host rhyolite and diorite. The similarity of 143Nd/144Nd vs 147Sm/144Nd and εNd vs P2O5 in the 1st- and 2nd-generations of apatite and the host rocks indicated that recrystallization of the apatites occurred during the magmatic and hydrothermal fluids circulation which were derived from the felsic to intermediate subvolcanic rocks. Difference in the age of the 2nd-generation apatites and the paragenetic- monazites ( 238U/206Pb and 207Pb/206Pb dating), the crystalline apatites and magnetite, the ilmenite exclusions in the magnetites, the dissolution evidences of different apatites and monazites generations, the content of Ti vs V, Al+Mn vs Ti+V and Mg+Al+Si vs Ti, and the O-H isotopes of the magnetite-apatite ores, all indicate the mixing of high-temperature magmatic and hydrothermal fluids rich in REE, P with Ca ±Fe evaporatic brines in different time periods, which caused a polygenic origin for the Esfordi deposit. Introduction The origin of the magnetite-apatite ore deposits in the Bafq mining district has been explained by a variety of mineralization models, including: a) metasomatic-hydrothermal (IOCG) related to Kiruna-type iron ore deposits (Mehrabi et al., 2019; Ziapour et al., 2021), b) orthomagmatic Kiruna-type (Mehdipour Ghazi et al., 2020; Vesali et al., 2021), and c) Ediacarian-paleoglacial BIF (Aftabi et al., 2021). This study combines evidence from mineralogy, geochemistry, stable isotopes, and apatite and magnetite ores from the Esfordi ore deposit to investigate the origin of mineralizing fluids for the first time. The results of this research could be used to explain the mineralization mechanisms of magnetite-apatite ore deposits in the Bafq mining district. Materials and methods Twenty samples of crystalline apatite of the first and second generations, twenty-two samples of massive fine grained apatite ore, and twenty-three magnetite-apatite samples were collected from different ores sections. Petrographic and mineralogical studies were carried out on 47 microscopic thin sections. Scanning Electron Microscopy (SEM) (18 samples) and XRD analyses (7 samples) were used to analyze the representative samples. ICP-OES and ICP-MS techniques were used at the Iranian Mineral Processing Research Center to analyze representative samples from apatite ores (12 samples), magnetite ores (12 samples), hematite ores (2 samples), jaspilite (10 samples), rhyolite (6 samples), rhyolitic tuff (5 samples), and metasomatized host rocks (5 samples). Fluid inclusion investigations on Twelve apatite crystals were conducted at Tehran's Zaminriz Kavan Research Company and the Geological Survey of Iran. Six samples of apatite ore were submitted to Hungaria laboratory for O-H isotopic analysis, and three samples were sent to Queensland University in Australia for Nd-Sm isotope analysis in order to conduct the isotopic analysis. Laser Ablation Coupled Plasma Mass Spectrometry was also used at Tasmania University in Australia to analyze four samples of apatite ores. Results This research reveals the Esfordi apatite ores are derived from the sedimentary phosphorites. The O-H isotopic data and the Sr and Mn content of the first and second generations as well as the massive fine-grained apatites, display the role of evaporitic brines in their formation. According to the contents of 143Nd/144Nd vs 147Sm/144Nd and εNd vs P2O5, as well as the variety in Nd isotopic ratios, the massive fine grained apatites, which forms the majority of the apatite mineralization (>95%), lacks a clear genetic relationship in terms of provenance with the rhyolitic, dioritic, and microdioritic host rocks. The similarity of 143Nd/144Nd vs 147Sm/144Nd and εNd vs P2O5 in the first and second generations of apatites and the host rocks demonstrated that the recrystallization of apatite rocks occurred under the influence of magmatic and hydrothermal fluids originating from the felsic to intermediate subvolcanic rocks in the area, which resulted in an increase in εNd values. The differences in age between the second-generation apatite and the paragenetically related monazites, using 238U/206Pb and 207Pb/206Pb dating methods, besides dissolution evidence in different generations of apatites and monazites, Ti vs V, Al+Mn vs Ti+V and O-H isotopes of the magnetite-apatite ores, indicated the role of high temperature magmatic and hydrothermal fluids along with evaporitic brines in mineralization in different time spans. This processes lead to a diversity of mineralization and a polygenic origin for the Esfordi apatite-magnetite ore deposit. Discussion The Esfordi ore deposit contains three different forms of apatites mineralization, including vein-type, fine grained massive and disseminated ores according to field observations. There were five generations of apatite, according to petrographic data. Numerous rare earth element minerals, including alanite, parisite-synchysite, bastenasite, and britolite, as well as two generations of monazite and one generation of limited xenotime, were identified in the Esfordi ore deposit according to investigations on the first and second generation apatites. Stable H-O and radiogenic Nd-Sm isotopic studies on the first and second generation apatites and massive fine grained apatite ores along with the similarity between εNd contents in apatite and phosphorites in Soltanieh Formation and phosphorite nodules of the Eastern European platform (Ediacarne and Lower Cambrian deposits) as well as Lower Cambrian sedimentary phosphate deposits in Siberia, Western Mongolia, Baltic, South Kazakhstan, South China, Australia, West Newfoundland, North Greenland and East Greenland confirms that the investigated apatites were formed from leaching of old or contemporaneous sedimentary phosphorites of Soltanieh Formation while magmatic and hydrothermal fluids originated from granitoid masses circulated in massive fine grained apatite ores. By the way, these crystalline apatites have been enriched in εNd content under the influence of magmatic and hydrothermal fluids originated from deep to sub-volcanic felsic and intermediate intrusions in this region. Investigation using the radiometric dating methods (238U/206Pb and 207Pb/206Pb) on the second-generation apatite and the paragenetically related monazites showed that these minerals were formed between 494-528 Ma and 514-556 Ma, respectively. Some monazites are older than apatites (approximately 28 Ma), which indicates that they were formed before apatite ore and it has been affected by hydrothermal fluids in the structure of apatite. The dating, for a limited number of monazites, indicates a time span between 23 to 33 and a time span of 104 to 153 Ma. The age differences between the apatite and monazite inclusions can be due, not only, to late alteration of deep to sub-volcanic bodies originated hydrothermal fluids, but also, to separation of U-Pb from this system or the formation of young monazites during orogenesis in different time spans. The presence of recrystalized apatite and magnetite, zoning and dissolution evidence in some monazites, dendritic texture in actinolite, ilmenite exsolutions and stable isotopes of magnetite and apatite ores indicates the mixing of magmatic and high temperature hydrothermal fluids with evaporatic brines enriched in REE, P, Ca ±Fe resulting in a diversity of processes involved in formation of the Esfordi ore deposit. Acknowledgements The authors appreciate Shiraz University Research Council for support of this work. The Director General and personal of the Esfordi Mine Company are acknowledged for their assistance in the field works.

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